Black Holes in General Relativity and Beyond

Enrico Barausse

doi:10.3390/proceedings2019017001

Abstract

The recent detections of gravitational waves from binary systems of black holes are in remarkable agreement with the predictions of General Relativity. In this pedagogical mini-review, I go through the physics of the different phases of the evolution of black hole binary systems, providing a qualitative physical interpretation of each one of them. I also briefly describe how these phases would be modified if gravitation were described by a theory extending or deforming General Relativity, or if the binary components turned out to be more exotic compact objects than black holes.

Highlights

The binary black hole (BH) systems detected by ground based gravitational wave (GW)interferometers are ten, and their signals have been found to be in excellent agreement with the predictions of GeneralRelativity (GR)
I do so by conventionally splitting the signal into four parts: an early low-frequency portion produced by the binary’s inspiral (Section 2); one produced by the merger (Section 3) with the subsequent ringdown phase (Section 4); and a post-ringdown phase produced by putative exotic near horizon physics (Section 6). (This fourth phase is absent in GR.) I qualitatively interpret each part by relying on physical intuition from the test particle limit, Newtonian physics and BH
I have tried to show how the GW signal from the merger of two BHs can be understood qualitatively based on very simple physical ingredients, including the quadrupole formula, which allows for understanding the low frequency inspiral and its dependence on the chirp mass; the PN precession of the spins during the inspiral; the presence of an effective ISCO, whose position depends on the BH spins as a result of “frame dragging” and which affects the overall power emitted in GWs; and the circular photon orbit and its effective potential, whose physics determines the QNMs

Summary

Introduction

The binary black hole (BH) systems detected by ground based gravitational wave (GW). interferometers (first by the two LIGOs alone [1,2,3,4,5], by the LIGO–Virgo network [6,7]) are ten, and their signals have been found to be in excellent agreement with the predictions of General. While GR had previously passed experimental tests with flying colors in weak field and/or mildly relativistic regimes (in the solar system [8] and in binary pulsars [9,10]), the LIGO-Virgo detections provide for the first time evidence that GR is viable in the strong gravity, highly relativistic and dynamical regime relevant for BH binaries [3,11] They support the hypothesis that the binary’s components, as well as the merger remnant, are really BHs, as opposed to more exotic compact objects [1]. I do so by conventionally splitting the signal into four parts: an early low-frequency portion produced by the binary’s inspiral (Section 2); one produced by the merger (Section 3) with the subsequent ringdown phase (Section 4); and a post-ringdown phase produced by putative exotic near horizon physics (Section 6). (This fourth phase is absent in GR.) I qualitatively interpret each part by relying on physical intuition from the test particle limit, Newtonian physics and BH perturbation theory

The Inspiral

The Merger

The Ringdown

The Post-Ringdown

Findings

Conclusions